The present invention relates to a head box for a paper making machine, cardboard making machine, or the like, having a plurality of flow channels and particularly to means for improving turbulence in the pulp.
For decades, efforts have been made to improve the head boxes so that the jet of pulp emerging from the head box has a uniform velocity distribution and so that the finest possible uniform distribution of the fibers within the stream of pulp is assured.
Until now, this purpose has been substantially achieved with various measures. The constructions described in the following publications have been particularly satisfactory: Federal Republic of Germany AS 2 007 308 (which is similar to U.S. Pat. No. 3,769,155), and Federal Republic of Germany AS 2 308 849 (which corresponds to U.S. Pat. No. 3,945,822). In accordance with these constructions, the fiber suspension of pulp in the head box is conducted through a plurality of channels, which are distributed over the width of the machine. These channels are relatively long and of small inside diameter. In this way, so-called microturbulence is produced by wall friction of the pulp in each of the channels. The intensity of the turbulence is generally so great that any fiber flocculations formed are again broken down or are kept so small that, as a rule, they are not detrimental to the finished paper. At the same time, the channels gradually widen continuously before discharging into the outlet nozzle. As a result, the entire flow cross-section available increases only slightly, if at all, at the point of discharge from the channels into the outlet nozzle, so that the so-called entrance surge is kept small and the occurrence of eddies in the outlet nozzle is avoided. (If the channels converge toward each other in the direction of flow, as seen in side view, then it may be that the channels, as seen in this side view, do not widen continuously. In all cases, however, there is a continuous widening of the channels when the plurality of channels is seen in top view.)
One prerequisite for the good action of these known constructions is that a relatively high flow velocity be provided in the channels and in the outlet nozzle. These known head boxes are therefore particularly suitable for a high speed paper making machine. However, difficulties occur when such a head box is to be used on a machine of relatively low operating speed. In that case, the number of channels (seen in side view) and the inside diameter of each of these channels can be reduced only within narrow limits. There is still the possibility of feeding the fiber suspension in more highly diluted form to the head box. However, this requires a higher expenditure of energy for circulating the pulp water, which requires disagreeably large pumps and pipelines.